Digital center tracking system

Abstract

A digital servo system for maintaining a detection device, such as a light beam, centered as closely as possible over an information track, such as an optical track on a video recording disc. The servo system includes circuitry for detecting when the light beam traverses a switching line, which, in the illustrative embodiment, is the track centerline, for applying a constant-magnitude acceleration force to deflect the beam to a corrected position, and for reversing the polarity of the acceleration force when the beam traverses the switching line.

I claim: 
     
       1. A digital servo control system for controlling the position of detection means with respect to the centerline of an information track without dithering the detection means, said system comprising: first transducer means for moving the detection means transversely with respect to the information track;   second transducer means for deriving an information signal from the detection means;   means for filtering the information signal to obtain an error signal indicative of the position of the detection means with respect to the track centerline;   signal processing means, for processing the error signal to obtain a timing signal indicative of transitions of the detection means through a switching line parallel with the track; and   control means, responsive to the timing signal, for controlling the polarity and duration of an acceleration signal applied to said first transducer means to accelerate the detection means toward a corrected position;   whereby the detection means is accelerated in alternating directions and oscillates about the track centerline, solely under the influence of the acceleration signal applied to accelerate the detection means toward a corrected position.   
     
     
       2. A digital servo control system as set forth in claim 1, wherein: the switching line is coincident with the track centerline;   the timing signal generated by said signal processing means changes state each time that the detection means crosses the centerline; and   said control means reverses the polarity of the acceleration signal each time that the timing signal changes state, to keep the acceleration always directed toward the centerline.   
     
     
       3. A method for controlling the position of detection means with respect to the centerline of a longitudinal information track, without dithering the detection means, said method comprising the steps of: deriving an information signal from the detection means;   filtering the information signal to obtain an error signal indicative of the position of the detection means with respect to the track centerline;   processing the error signal to obtain a timing signal indicative of transitions of the detection means through a longitudinal switching line;   generating an acceleration signal for application to the detection means;   controlling the polarity and duration of the acceleration signal in accordance with the timing signal; and   applying the resulting acceleration signal to the detection means, to move it in an oscillatory fashion about the track centerline.   
     
     
       4. A method as set forth in claim 3, wherein: the switching line is coincident with the track centerline;   the timing signal obtained in said processing step changes state each time that the detection means crosses the centerline; and   said controlling step includes reversing the polarity of the acceleration signal each time that the timing signal changes state, whereby the detection means is constantly accelerated toward, and oscillates about, the centerline.   
     
     
       5. A digital servo control system for controlling the position of a reading beam used to derive information from a spiral track on a recording disc, without dithering the reading beam, said system comprising: beam position transducer means, for moving the beam transversely with respect to the information track;   
     
     
       detection transducer means, for deriving an information signal from the beam; means for filtering the information signal to obtain an error signal indicative of the beam position with respect to the track centerline;   signal processing means, for processing the error signal to obtain a timing signal indicative of transitions of the beam through a longitudinal switching line along the information track; and   control means, responsive to the timing signal, for controlling the polarity and duration of an acceleration signal applied to said beam position transducer means to accelerate the beam toward a corrected position, whereby the beam is accelerated in alternating directions and oscillates about the track centerline, solely under the influence of the acceleration signal applied to accelerate the beam toward a corrected position.   
     
     
       6. A digital servo control system as set forth in claim 5, wherein: the switching line is coincident with the track centerline;   the timing signal produced by said signal processing means changes state each time that the beam crosses the centerline; and   said control means reverses the polarity of the acceleration signal on each crossing of the centerline, and maintains a constant acceleration of the beam toward the centerline.   
     
     
       7. A digital servo control system as set forth in claim 6, wherein: the error signal is approximately sinusoidal, with peaks in one direction corresponding to the times of track centerline crossings; and   said signal processing means includes means for shifting the error signal in phase by ninety degrees to obtain an intermediate signal that makes sign transitions at times corresponding to peaks in the error signal, and means for generating the timing signal from the intermediate signal, in such a manner that the timing signal has sign transitions only at times corresponding to the track centerline crossings.   
     
     
       8. A digital servo control system as set forth in claim 7, wherein said control means includes switching means for selecting the polarity of the acceleration signal as determined by the transitions in the timing signal.